(1)\MSH-binding proteins were isolated from murine melanoma cells by affinity chromatography. Three bands (200K, 110K, 90K) could be detected by SDS-PAGE and by isoelectric focusing (pI=5.3, 5.7, 5.8). There was no significant difference in either the amounts or the physical characteristics of MSH-binding proteins obtained from cells treated with neuraminidase. (2)\We have demonstrated that MSH inhibits DNA nicking-and-closing enzyme (topoisomerase I). The inhibition is specific for MSH; however, it occurs at a relatively high (greater than 10 micromolar) concentration. (3)\We have investigated the intracellular fate of 125I-MSH by isolation of radioactive components using HPLC, following hormone internalization at 37~C. We found that the native hormone was quickly converted into several smaller and one larger molecular species of unknown chemical identity. (4)\Chemically homogenous conjugates of MSH-daunomycin and MSH-SPDP were isolated by HPLC and characterized by amino acid analysis in order to be used for MSH receptor-mediated targeting of drugs. (5)\Cell cultures of normal human melanocytes were initiated and maintained for more than 1 year by using neonatal human foreskins as the source of cells. The ganglioside composition of normal melanocytes and human melanoma cells were found to be significantly different, i.e., melanoma cells contained 10 to 20 times more GD3 than did normal melanocytes. The difference in ganglioside composition is being exploited for nuclear imaging of melanoma tumors using radiolabeled monoclonal antibodies that are specific for ganglioside GD3. (N)